Multi-band amplification system for dense wavelength division multiplexing

ABSTRACT

An optical transmission system has been designed to optimize the use of the spectral emission range of rare-earth-doped fiber amplifiers. The system includes a wide band of channels in the spectral emission range of erbium-doped fiber amplifiers, which is split into two sub-bands, a low sub-band corresponding to the low end of the range and a high sub-band corresponding to the high end of the range. The two sub-bands are separately amplified and optimized, and then recombined without significant competition between the two sub-bands. In addition, an equalizing filter, such as a specialized Bragg filter or interferential filter, is applied to the low sub-band instead of the entire band of channels, thus greatly reducing any equalization need or unequalization effects. In an optical line amplifier for the system, the wide band is amplified in a first stage of a fiber amplifier that operates in a linear condition, the wide band is split into the two sub-bands, and one or the two sub-bands is amplified with the second stage of the fiber amplifier that operates in a saturation condition.

This application is a continuation of International Application No.PCT/EP98/03967, filed Jun. 29, 1998, the content of which isincorporated herein by reference and claims the benefit of U.S.Provisional Application No. 60/055,065, filed Aug. 8, 1997.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of wavelengthdivision multiplexing within optical transmission systems, and moreparticularly to the field of wavelength division multiplexing using bandseparation within a generic spectral emission range of arare-earth-doped fiber amplifier.

In optical transmission systems, optical fibers doped with rare-earthelements such as erbium provide a useful component for amplifyingsignals passing across a long distance link. These fiber amplifiers,when pumped with a first characteristic wavelength, provide gain to atransmission signal at a second characteristic wavelength. When erbiumis used as the rare-earth dopant, the pump wavelength typically iseither 980 nm or 1480 nm, which results in a stimulated emissionspectrumfor the amplifier across a band of about 1528-1562 nm. Therefore, theerbium-doped fiber amplifier will amplify transmission signals passingthrough it at these wavelengths.

An optical transmission system using erbium-doped fiber amplifiers,however, suffers from several limitations due to the emissioncharacteristics of the amplifier across the wavelength range. For one,the spectral emission of the erbium fiber is non-flat across thewavelength band of 1528-1562. As a result, only a narrow band ofwavelengths have conventionally been used to obtain equivalent gainacross the band. Many systems have chosen 1550 nm and its surroundingwavelengths as the narrow band due to the relatively flat response ofthe erbium-doped fiber amplifier in this region. When a high number ofchannels using dense wavelength division multiplexers (WDM) are appliedto the erbium-doped fiber amplifier, techniques such as equalizing meansmust be employed in an attempt to flatten the gain of the amplifieracross the bandwidth of the system. These equalizing means encumbersystem design. In addition, the cascading of amplifiers in a large WDMsystem compounds the issues with non-flat gain and imposes furtherfundamental limitations on system design.

FIG. 1 is a graph of a generalized spectral emission range of 1528-1562nm for an erbium-doped fiber amplifier showing the different gain forchannels of signals traveling through an optical communication linkincluding the erbium-doped fiber. As shown in FIG. 1, the gain in alower region between 1528 nm and 1541 nm is non-flat, whereas the gainin the higher region is mostly flat. In WDM systems, discretewavelengths within a small tolerance, otherwise known as channels, areused to carry modulated information. For channels in the lower region,the disparity in gain for signals passing through an erbium-doped fiberamplifier may cause unequal amplification among the channels. Thedisparity becomes more significant when the channels pass though acascade of amplifiers that have similar gain characteristics. Thedifferences in gain among the channels can become extreme enough tocause channels with very low gain to fall below a predetermined noisecutoff level. The performance specifications of a receiver positioneddownstream from the amplifiers may dictate the noise cutoff level.Channels falling below the noise cutoff are not detected, or detectedpoorly, effectively eliminating those channels or reducing theirreliability.

To overcome the gain disparity problems, optical transmission systemshave used equalizing devices such as notch filters as a dual core fiber,interferential filters, long period grating, chirped gratings, or hybridactive fiber, for example, to flatten the gain characteristic. Some ofthese techniques are discussed in U.S. Pat. No. 5,260,823. However,these equalizing devices are only effective in limited applications,such as linear conditions, and are thus liable to maintain continuedgain disparities when applied to the erbium-doped fiber spectralemission range of 1528-1562 nm. Thus, due to the potentially large gaindisparities in the lower channel region and the corresponding problemsof flattening the gain characteristics, optical transmission systemshave been limited to using the higher end of the erbium-doped fiberamplifier spectral emission range.

U.S. Pat. No. 5,392,154 proposes a self-regulating multiwavelengthoptical amplifier module providing desired channel-by-channel powerregulation and immunity to transient interchannel cross-saturation. Theproposed amplifier module includes a plurality of pump-shared parallelfiber amplifiers operated in gain-saturation and connected between ademultiplexer and a multiplexer. Each of the fiber amplifiersindividually amplifies one channel at a single wavelength. An optionalfirst gain stage comprising a strongly pumped erbium-doped fiberamplifier improves performance with higher optical signal-to-noiseratio.

E.P. Patent Application No. 445,364A proposes an optical fibercommunication system providing a connection between a central stationand a number of subscriber stations and including an optical amplifier(OV) having wavelength selective couplers at input and output adapted todirect a first wavelength λ1 into the amplifier and a second wavelengthλ2 into a bridging conductor (U).

U.S. Pat. No. 5,452,116 discloses a wavelength division multiplexedoptical transmission system incorporating a concatenation of opticalamplifiers. The multiplexed signal passes through a limited number ofamplifiers in which all channels are amplified together. Then, thesignal is demultiplexed and the individual channels are separatelyamplified and then remultiplexed together. In instances where a set ofchannels may be grouped into subsets of channels for which theindividual channel spacing is so close that any differentialamplification is negligible, then the set may be amplified separatelyfrom another set.

Similarly, U.S. Pat. No. 5,608,571 discloses an optical amplifier for aWDM system that has a set of optically amplifying fibers arranged withan associated spectrally selective Bragg reflector. Different spectralcomponents of an input signal propagate through different ones of theoptically amplifying fibers based on the reflection band of theassociated Bragg reflectors and return to a transmission path.

U.S. Pat. No. 5,563,733 discloses an apparatus for optically amplifyinga plurality of signals having different wavelengths where a first signalamong the plurality propagates through a part of a series ofrare-earth-doped optical fibers and a second signal among the pluralitypropagates through all of the series of rare-earth-doped optical fibers.The disclosed arrangement aims to provide an equalizing gain for signalshaving different input powers, for example a digital signal that has asmall input power and an analog signal that has a large input power. AWDM coupler separates the analog signal before it passes through all ofa series of fiber amplifiers.

Applicants have discovered that the limited region in the erbium-dopedfiber amplifier spectral emission range used for transmitting signalsdoes not fulfill the needs of dense WDM systems, particularly WDMsystems having sixteen or more channels and using erbium-doped fiberamplifiers. Applicants have found that the prior arrangements ofseparating certain types of signals from a cascade of amplifiers orseparately amplifying groups of channels having negligible differentialamplification fall short of fulfilling the needs of dense WDM systems.

Applicants have observed that prior art approaches. may suffer fromhaving the power of individual output channels not be independent fromthe other channels. Moreover, Applicants have observed that whenband-separated equalizing techniques are employed within the stages ofamplification in a WDM system, the power of the channels as well as thespectra can be effectively separated and made independent. In this way,relatively consistent output power between channels of a dense WDMsystem can be obtained, and the power performance of the channels of asub-band can be made relatively independent of the presence or absenceof channels in other sub-bands.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical transmission systemhas been designed to optimize the use of the erbium spectral emissionrange. The system includes a wide band of channels in the erbiumspectral emission range, which is split into two sub-bands-a lowsub-band corresponding to the low wavelength end of the range and a highsub-band corresponding to the high wavelength end of the range. The twosubbands are separately amplified and optimized, and then recombinedwithout significant competition between the two sub-bands and withreduced gain tilt effects. In addition, an equalizing filter, such as aspecialized Bragg filter like a long period grating, or aninterferential filter is applied to the low sub-band instead of theentire band of channels, thus greatly reducing any equalization need orunequalization effect.

To obtain the advantages and in accordance with the purpose of theinvention, as embodied and broadly described herein, an opticalcommunication system for transmitting optical channels between atransmitter and a receiver using wavelength division multiplexingincludes a wavelength multiplexer optically coupled to the transmitterfor multiplexing individual optical channels, a transmitter poweramplifier optically coupled to the wavelength multiplexer for amplifyingthe multiplexed optical channels, at least one optical line amplifierbeing optically coupled to the transmitter power amplifier via anoptical transmission fiber, a receiver power amplifier optically coupledto the at least one line amplifier via another optical transmissionfiber, and a wavelength demultiplexer optically coupled to the receiverpower amplifier far separating the multiplexed optical channels into theindividual optical channels for passage to the receiver. The opticalline amplifier includes a first stage of a first fiber amplifier foramplifying the multiplexed optical channels, a first band separationfilter optically coupled to an output of the first stage for splittingthe multiplexed optical channels into a first band of wavelengths and asecond band of wavelengths, a second stage of the first fiber amplifieroptically coupled to the band separation filter, a second fiberamplifier optically coupled to the band separation filter and having afirst wavelength response characteristic for amplifying the first band,an equalizing filter positioned between the band separation filter andthe second amplifier for equalizing the amplification of signals in thefirst band, a third fiber amplifier optically coupled to the secondstage and having a second wavelength response characteristic differentfrom the first wavelength response characteristic for amplifying thesecond band, and a combiner for multiplexing the first amplified bandand the second amplified band back into the multiplexed opticalchannels.

In addition, an optical line amplifier for amplifying a plurality ofmultiplexed channels traveling in a wavelength division multiplexingsystem includes a first optical amplifier, optically coupled to receivethe multiplexed channels, having a first stage operating in a linearmode and a second stage operating in a saturation mode; and a bandseparation filter positioned between the first stage and the secondstage for passing a first group of the multiplexed channels into thesecond stage and separating a second group of the multiplexed channelsfrom entering the second stage; a second optical amplifier, opticallycoupled to an output of the second stage, having a first wavelengthresponse characteristic for amplifying the first group of themultiplexed channels; a third optical amplifier, optically coupled tothe band separation filter, having a second wavelength responsecharacteristic different from the first wavelength responsecharacteristic for amplifying the second group of the multiplexedchannels; and an equalization filter positioned between the bandseparation filter and the third optical amplifier for flattening thegain response of the third optical amplifier for the second group of themultiplexed channels.

Furthermore, a method for transmitting optical signals, includes thesteps of amplifying a multiplexed signal having a plurality of opticalchannels with a first stage of a first amplifier operating in a linearcondition; splitting the multiplexed signal into a first wavelengthband; and a second wavelength band, and amplifying the first wavelengthband with a second stage of the first amplifier operating in asaturation condition; amplifying the first wavelength band after thesecond stage with a second amplifier having a first wavelength responsecharacteristic; filtering the second wavelength band to flatten a gainresponse; and amplifying the second wavelength band with a thirdamplifier having a second wavelength response characteristic differentfrom the first wavelength response characteristic.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed. The followingdescription, as well as the practice of the invention, set forth andsuggest additional advantages and purposes of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, explain the advantages and principles ofthe invention.

FIG. 1 is a graph of a gain characteristic for an erbium-doped fiberspectral emission range;

FIG. 2 is a block diagram of an optical transmission system consistentwith the present invention;

FIG. 3 is a graph of a gain characteristic for an erbium-doped fiberspectral emission range, as in FIG. 1, with a designation of a low bandand a high band;

FIG. 4 is a more detailed diagram of the multiplexing section of theoptical transmission system in FIG. 2;

FIG. 5A is a more detailed diagram of the transmitter power amplifiersection of the optical transmission system in FIG. 2;

FIG. 5B is a graph of a filter performance shape of a de-emphasis filterfor the transmitter power amplifier of the present invention;

FIG. 6 is a more detailed diagram of the optical line amplifier sectionof the optical transmission system in FIG. 2;

FIGS. 7A and 7B are graphs of an insertion loss characteristic of anequalizing filter and the corresponding gain characteristic for anerbium-doped fiber amplifier, respectively;

FIG. 8A is a graph of an experimental output of an optical transmissionsystem consistent with the present invention with six channels in a lowband;

FIG. 8B is a graph of an experimental filter performance shape of anequalizing filter using long period Bragg grating technology;

FIG. 8C is a graph of an experimental output of the optical transmissionsystem with six channels in the low band of FIG. 8A, with the equalizingfilter with the filter shape of FIG. 8B coupled in the low band secondstage of the line amplifiers in the system;

FIG. 8D is a graph of an experimental output of the optical transmissionsystem of FIG. 8A with eight channels in a low band;

FIG. 8E is a graph of an experimental output of the optical transmissionsystem of FIG. 8D with eight channels in the low band of FIG. 8D, withthe equalizing filter with the filter shape of FIG. 8B coupled in thelow band second stage of the line amplifiers in the system;

FIG. 9 is a more detailed diagram of the receiver pre-amplifier sectionof the optical transmission system in FIG. 2;

FIGS. 10A and 10B are more detailed diagrams of the demultiplexingsection of the optical transmission system in FIG. 2;

FIG. 11A is an experimental result of a WDM system consistent with thepresent invention of the high band of twenty-one channels;

FIG. 11B is an experimental result of a WDM system consistent with thepresent invention of the low band of six channels;

FIG. 11C is an experimental result of a WDM system consistent with thepresent invention of the wide band of twenty-seven channels; and

FIG. 12 is a chart of preferred attenuation for various spans and systemconfigurations for the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to various embodiments according to thisinvention, examples of which are shown in the accompanying drawings andwill be obvious from the description of the invention. In the drawings,the same reference numbers represent the same or similar elements in thedifferent drawings whenever possible.

This invention relates to an optical transmission system that optimizesthe use of the spectral emission range of an erbium-doped fiberamplifier. The following describes the general layout of the opticaltransmission system consistent with the present invention. Referring toFIG. 2, optical transmission system 100 includes a multiplexing section(MUX) 110, a transmitter power amplifier (TPA) section 120, at least twosections of longdistance optical transmission fiber (not shown), anoptical line amplifier (OLA) section 130 positioned between every twosections of optical transmission fiber, a receiver pre-amplifier (RPA)section 140, and a demultiplexing section (DMUX) 150. Opticaltransmission system 100 further includes a plurality of input channels160 and a plurality of output channels 170.

Input channels 160 may, for example, include 8, 16, or 32 channels, eachhaving a distinct carrier wavelength, or some other total of channelsdepending on the needs and requirements of the particular opticaltransmission system. Each input channel 160 is received by multiplexingsection 110. As shown in FIG. 2, multiplexing section 110 multiplexes orgroups input channels 160 preferably into two sub-bands, althoughmultiplexing section 110 could alternatively group input channels 160into a single wide-band or a number of sub-bands greater than two.

The two preferable sub-bands produced by multiplexing section 110 arethen received, as separate sub-bands or as a combined wide-band, insuccession by TPA section 120, at least one OLA section 130, and RPAsection 140. Sections of optical transmission fiber (not shown) adjointhe at least one OLA section 130 with TPA section 120, RPA section 140,and possibly with other OLA sections (not shown). TPA section 120receives the separate sub-bands from multiplexing section 110, amplifiesand optimizes them, and then combines them into a single wide-band. Afirst section of optical transmission fiber (not shown) couples theoutput of TPA section 120 to OLA section 130. OLA section 130 receivesthe single wide-band and re-divides it into the two sub-bands. OLAsection 130 amplifies and optimizes the two sub-bands and thenrecombines them into the single wide-band. A second section of opticaltransmission fiber (not shown) couples the output of the OLA section 130to either another OLA section (not shown) or to RPA section 140. RPAsection 140 also amplifies and optimizes the single wide-band and maysplit the single wide-band into the two sub-bands before outputtingthem.

Demultiplexing section 150 then receives the two sub-bands from RPAsection 140. Demultiplexing section 150 splits the two sub-bands intothe individual wavelengths of output channels 170. In general, thenumber of output channels 170 will be the same as the number of inputchannels 160. However, some channels may be either added or dropped byoptical transmission system 100 between multiplexing section 110 anddemultiplexing section 150 by an optical add/drop multiplexer (OADM),described further below. Consequently, the number of input channels 160and output channels 170 may be unequal in some circumstances.

As shown in FIG. 2, multiplexing section 110 preferably groups inputchannels 160 into two sub-bands. Both sub-bands are within the spectralemission range of the optical fiber amplifiers used in TPA 120, OLA 130,and RPA 140. In a preferred embodiment, the fiber amplifiers in thosesections of WDM system 100 are erbium-doped fiber amplifiers. As aresult, the two sub-bands fall between 1528 nm and 1562 nm. One sub-bandis referred to as a low band (LB) and the other as a high band (HB).

FIG. 3 is another graph of the erbium-doped fiber amplifier spectralemission range of 1528-1562 nm, which generally shows the different gainfor channels of signals traveling through the erbium-doped fiber link.As shown in FIG. 3, the spectrum is divided in MUX 110 into two rangesthat correspond to the low band (LB) and the high band (HB). Inparticular, the low band preferably covers the range between 1529 nm and1535 nm, and the high band preferably covers the range between 1541 nmand 1561 nm. The gain characteristic in the high band is fairly flat,but the low band includes a substantial hump in the gain response. Asexplained below, to make use of the erbium-doped fiber spectral emissionrange in the low band, optical transmission system 100 uses equalizingmeans to flatten the gain characteristic in that range. As a result, bydividing the entire erbium-doped fiber spectral emission range of1528-1562 nm into two sub-ranges that correspond to the low band andhigh band, optical transmission system 100 can effectively use most ofthe erbium-doped fiber spectral emission range and provide for denseWDM.

The following provides a more detailed description of the variousmodules of the present invention depicted in FIG. 2. Referring to FIG.4, a more detailed diagram of multiplexing section 110 of opticaltransmission system 100 illustrates optical line terminal section (OLTE)410, a wavelength converter section (WCS) 420, and two wavelengthmultiplexers (WM) 430 and 440. OLTE 410, which may correspond tostandard line terminating equipment for use in a SONET or SDH system,includes transmit/receive (TX/RX) units (not shown) in a quantity thatequals the number of channels in WDM systems 100. In a preferredembodiment, OLTE 410 has thirty-two (32) TX/RX units. As readilyunderstood to one of ordinary skill in the art, OLTE 410 may comprise acollection of smaller separate OLTEs, such as two, that feed informationfrequencies to WCS 420. Accordingly, WCS 420 includes thirty-two (32)wavelength converter modules WCM1-WCM32.

In multiplexing section 110, OLTE 410 transmits a plurality of signalsat a generic wavelength. As shown in FIG. 4, for a preferred embodiment,OLTE 410 outputs a grouping of eight (8) signals and a grouping oftwenty-four (24) signals. However, as indicated above, the number ofsignals may vary depending on the needs and requirements of theparticular optical transmission system. Units WCM1-WCM8 each receive oneof the grouping of eight signals emitted from OLTE 410, and unitsWCM9-WCM32 each receive one of the grouping of twenty-four signalsemitted from OLTE 410. Each unit is able to convert a signal from ageneric wavelength to a selected wavelength and re-transmit the signal.The units may receive and re-transmit a signal in a standard format,such as OC-48 or STM-16, but the preferred operation of WCM1-32 istransparent to the particular data format employed.

WCM1-32 preferably comprise a module having a photodiode (not shown) forreceiving an optical signal from OLTE 410 and converting it to anelectrical signal, a laser or optical source (not shown) for generatinga fixed carrier wavelength, and an electro-optic modulator such as aMach-Zehnder interferometer (not shown) for externally modulating thefixed carrier wavelength with the electrical signal. Alternatively,WCM1-32 may comprise a photodiode (not shown) together with a laserdiode (not shown) that is directly modulated with the electrical signalto convert the received wavelength to the carrier wavelength of thelaser diode. As a further alternative, WCM1-32 comprises a module havinga high sensitivity receiver (e.g., according to SDH or SONET standards)for receiving an optical signal, e.g.; via a wavelength demultiplexer,from a trunk fiber line end and converting it to an electrical signal,and a direct modulation or external modulation laser source. By thelatter alternative, regeneration of signals from the output of a trunkfiber line and transmission in the inventive optical communicationsystem is made possible, which allows extending the total link length.WCM1-32 may be obtained, for example, from Applicants' assignee underthe abbreviation TXT, WCM, or LEM.

The selected wavelength for each WCM within WCS 420 is preferablydetermined according to a standard grid, for example and not by way oflimitation that shown in Table 1 below, such that each signal has adifferent wavelength. Each unit WCM1-WCM32 must be tuned and set toparticular tolerances as is known in the art. Of course, the frequencyseparation of channels depends upon the system implementation chosen andmay be, for example, 100 Ghz between each channel. Alternatively, thefrequency spacing may be unequal to alleviate four-wave-mixingphenomenon.

The channel allocation shown in Table 1 below is designed for both a 2.5Gb/s system and a 10 Gb/s system. In each of these two systems, bandseparation still occurs, but for different wavelengths depending onwhether the system is using, for example, 8, 16, or 32 channels.Although FIG. 4 shows the signals are provided and generated by thecombination of OLTE 410 and WCM1-WCM32, the signals can also be directlyprovided and generated by a source without limitation to their origin.

TABLE 1 Channel Allocation Nominal Channel System 2.5 Gb/s System 10Gb/s Thz nm 32(8 + 24) 16(4 + 12) 8(2 + 6) 8(low) 8(high) 16(high) 8(2 +6) 196   1529.55 L L 195.9 1530.33 L L L 195.8 1531.12 L L 195.7 1531.90L L L L L 195.6 1532.68 L L 195.5 1533.47 L L L L L 195.4 1534.25 L L195.3 1535.04 L L L 194.4 1542.14 H 194.3 1542.94 H H 194.2 1543.73 H194.1 1544.53 H H 194   1545.32 H H 193.9 1546.12 H H H H 193.8 1546.92H H 193.7 1547.72 H H H H H 193.6 1548.51 H H 193.5 1549.32 H H H H H H193.4 1550.12 H H H 193.3 1550.92 H H H H H H 193.2 1551.72 H H H 193.11552.52 H H H H H H 193   1553.33 H H H 192.9 1554.13 H H H H H 192.81554.94 H H 192.7 1555.75 H H H H H 192.6 1556.55 H H 192.5 1557.36 H HH H 192.4 1558.17 H 192.3 1558.98 H H 192.2 1559.79 H 192.1 1560.61 H H

Table 1 shows the nominal channel wavelengths output by the respectiveWCMs in WCS 420 for a WDM system 100 that uses up to thirty-two (32)channels. For a 2.5 Gb/s data rate, the third column lists theallocation between the low band and the high band for each of thethirty-two (32) channels generated by WCM1-32. The low band contains thefirst eight (8) channels, and the high band includes the nexttwenty-four (24) channels. Likewise, the fourth column shows the channelallocation for a sixteen (16) channel system with the four (4) channelsdesignated at the low band and twelve (12) channels as the high band. Ascan be seen for the sixteen (16) channel allocation, and for theremaining preferred channel allocations in Table 1, the systemimplementations using less than thirty-two (32) channels have greaterchannel-to-channel spacing across the same overall bandwidth. Asmentioned, the channel allocations in Table 1 illustrate a preferredselection for the channels within WDM system 100 and may be varied, bothwith respect to the individual channel wavelengths and the band for thechannel wavelengths, as system requirements dictate. For example, iffiber amplifiers are used that have a rare-earth dopant other thanerbium, or contain co-dopants in addition to erbium, the band of1528-1562 nm may shift, spread, or shrink. Likewise, the actual fiberamplifiers employed may more efficiently support a different allocationof channels between high and low bands to that shown representationallyin Table 1. As well, WDM system 100 may be revised or upgraded toaccommodate, for example, sixty-four (64) channels with 50 GHz spacingat 2.5 Gb/s or sixteen (16) channels with 100 GHz spacing at 10 Gb/s.

For the preferred thirty-two (32) channel system, each selectedwavelength signal output from units WCM1-WCM8 is received by WM 430, andeach selected wavelength signal output from WCM9-WCM32 is received by WM440. WM 430 and WM 440 combine the received signals of the twosub-bands, the low band and high band, respectively, into two wavelengthdivision multiplexed signals. As shown in FIG. 4, WM 430 is an eightchannel wavelength multiplexer, such as a conventional 1×8 planaroptical splitter, and WM 440 is a twenty-four channel wavelengthmultiplexer, such as a conventional 1×32 planar optical splitter witheight unused ports. Each wavelength multiplexer may include a secondport (e.g. 2×8 splitter and 2×32 splitter) for providing opticaltransmission system 100 with an optical monitoring channel (not shown).As well, WM 430 and 440 may have more inputs than is used by the system(e.g. 1×16 splitter and 1×64 splitter) to provide space for systemgrowth. A wavelength multiplexer using passive silica-on-silicon(SiO₂—Si) or silica-on-silica (SiO₂—SiO₂) technology, for instance, canbe made by one of ordinary skill in the art. Other technologies can alsobe used for WMs, e.g., for reducing insertion losses. Examples are AWG,gratings, and interferential filters.

The low band and high band output from multiplexing section 110 arereceived by TPA section 120. Naturally, the low band and high bandsignals may be provided to TPA section 120 from a source other than theOLTE 410, WCS 420, and WM 430 and 440 configuration depicted in FIG. 4.For example, the low band and high band signals may be generated anddirectly supplied to TPA section 120 by a customer without departingfrom the intent of the present invention described in more detail below.

As shown in FIG. 5A, TPA section 120 includes four amplifiers (AMP) 510,520, 530, and 540, and filters 550 and 560. Amplifiers 510 and 520 arearranged in series and amplify the low band, and amplifier 530, filter550, and amplifier 540 are also arranged in series and amplify the highband. The outputs of amplifiers 520 and 540 are received by filter 560,which combines the low band and the high band into a single wide-band(SWB).

Amplifiers 510, 520, 530, and 540 are preferably erbium-doped fiberamplifiers, although other rare-earth-doped fiber amplifiers may beused. Each of the amplifier 510, 520, 530, and 540 may be single-stageor multi-stage amplifiers as the particular design and system criteriawarrant Each amplifier is pumped, for example, by a laser diode toprovide optical gain to the signals it amplifies. The characteristics ofeach amplifier, including its length and pump wavelength, are selectedto optimize the performance of that amplifier for the particularsub-band that it amplifies. For example, with the preferred erbium-dopedfiber amplifiers, amplifiers 510 and 530 are pumped with a laser diode(not shown) operating at 980 nm to amplify the low band and high band,respectively, in a linear or in a saturated regime. Appropriate laserdiodes are available from Applicants assignee. The laser diodes may becoupled to the optical path of the amplifiers 510 and 530 using 980/1550WDM couplers (not shown) commonly available on the market, for examplemodel SWDMO915SPR from E-TEK DYNAMICS, INC., 1885 Lundy Ave., San Jose,Calif. (USA). The 980 nm laser diode provides a low noise figure for theamplifiers compared with other possible pump wavelengths.

The outputs of amplifiers 510 and 530 are received by amplifiers 520 and540, respectively. Amplifiers 520 and 540 preferably operate as boosteramplifiers in a saturated condition. Amplifier 520 amplifies the lowband with another 980 nm pump (not shown) coupled to the optical path ofthe low band using a WDM (not shown) described above. The 980 nm pumpprovides better gain behavior and noise figure for signals in the lowband region that covers 1529-35 nm. Amplifier 540 amplifies the highband preferably with a laser diode pump source operating at 1480 nm.Such a laser diode is available on the market, such as modelFOL1402PAX-1 supplied by JDS FITEL, INC., 570 Heston Drive, Nepean,Ontario (CA). The 1480 nm pump provides better saturated conversionefficiency behavior, which is needed in the high band for the greaternumber of channels in the region that covers 1542-61 nm. Alternatively,a higher power 980 nm pump laser or multiplexed 980 nm pump sources maybe used.

As shown in FIG. 5A, TPA section 120 may include filter 550 within thehigh band amplifier chain for helping to equalize signal levels and SNRsat the system output across the high band. In the preferred embodiment,filter 550 is positioned between the two amplifier stages formed by AMP530 and AMP 540 and comprises a de-emphasis filter that attenuates thewavelength regions of the high amplification within the high band. Thede-emphasis filter, if used, may employ long period Bragg gratingtechnology, split-beam Fourier filter, etc. As an example, thede-emphasis filter may have an operating wavelength range of 1541-1561nm and have wavelengths of peak transmission at 1541-1542 nm and1559-1560 nm, with a lower, relatively constant transmission for thewavelengths between these peaks. FIG. 5B illustrates the filter shape orrelative attenuation performance of a preferred de-emphasis filter 550.The graph of FIG. 5B shows that de-emphasis filter 550 has regions ofpeak transmission at around 1542 nm and 1560 nm, and a region ofrelatively constant or flat attenuation between about 1547 nm and 1556nm. The de-emphasis filter 550 for erbium-doped fiber amplifiers needonly have add an attenuation of about 3-4 dB at wavelengths between thepeaks to help flatten the gain response across the high band.De-emphasis filter 550 may have an attenuation characteristic differentfrom that depicted in FIG. 5B depending on the gain-flatteningrequirements of the actual system employed, such as the dopant used inthe fiber amplifiers or the wavelength of the pump source for thoseamplifiers. As well, de-emphasis filter 550 may be positioned along thepath of the high band at a location other than between AMP 530 and AMP540 as desired.

After passing through the amplifiers of TPA 120, the amplified low bandand high band output from amplifiers 520 and 540, respectively, arereceived by filter 560. Filter 560 may be, for example, aninterferential low pass three-port filter, which combines the low bandand high band into the single wide-band and outputs it from one commonport. Thus, filter 560 acts as a band combining filter. An opticalmonitor (not shown) and insertion for a service line, at a wavelengthdifferent from the communication channels, e.g. at 1480 nm, through aWDM 1480/1550 interferential filter (not shown) may also be added at thecommon port. The optical monitor detects optical signals to ensure thatthere is no break in optical transmission system 100. The service lineinsertion provides access for a line service module, which can managethrough an optical supervisory channel the telemetry of alarms,surveillance, monitoring of performance and data, controls andhousekeeping alarms, and voice frequency orderwire.

The single wide-band output from filter 560 of TPA section 120 passesthrough a length of transmission fiber (not shown), such as 100kilometers, which attenuates the signals within the single wide-band.Consequently, OLA section 130 is used to receive and amplify the signalswithin the single wide-band. As shown in FIG. 6. OLA section 130includes several amplifiers (AMP) 610, 615, 640, and 650, two filters620 and 660, and an equalizing filter (EQ) 630. Amplifier 610 receivesand amplifies the single wide-band, which is then separated back intothe low band and high band by filter 620. The low band is equalized byequalizing filter 630 and amplified by amplifier 640, whereas the highband, which has already passed through de-emphasis filter 550 in TPA120, is only amplified by amplifiers 615 and 650. The amplified low bandand high band are then recombined into the single wide-band by filter660.

Amplifier 610, which receives the single wide-band, preferably comprisesa single optical fiber amplifier that is operated in a linear regime.That is, amplifier 610 is operated in a condition where its output poweris dependent on its input power. Depending on the actual implementation,amplifier 610 may alternatively be a multi-stage amplifier. Applicantshave found that by operating it in a linear condition, amplifier 610helps to ensure relative power independence between the high bandchannels and the low band channels. In other words, with amplifier 610operating in a linear condition, the output power (and signal-to-noiseratio) of individual channels in the one of the two sub-bands does notvary significantly if channels in the other sub-band are added orremoved during operation of WDM system 100.

Consequently, the system of the present invention provides increasedflexibility in its application. More specifically, to obtain robustnesswith respect to the presence of some or all of the channels in a denseWDM system, the system consistent with the present invention shouldoperate with a first stage amplifier, such as amplifier 610, in anunsaturated regime in a line amplifying unit 130 before extracting aportion of the channels for separate equalization and amplification. Ina preferred embodiment, amplifier 610 is an erbium-doped fiber amplifierthat is pumped in a co-propagating direction with a laser diode (notshown) operating at 980 nm pump to obtain a noise figure preferably lessthan 5.5 dB for both the low band and high band.

Following amplifier 610, filter 620 receives the output from amplifier(AMP) 610 and splits the single wide-band into the low band and the highband components. Filter 620 may comprise, for example, a three-portdevice having a drop port that feeds the low band into equalizing filter630 and a reflection port that feeds the high band into amplifier 650 inFIG. 6. In this configuration, the filter 620 is preferably aninterferential filter that passes or drops the low band to equalizingfilter 630 with a high isolation of the high-wavelength part of thespectrum, and reflects the high band to amplifier 650 with a lowisolation of the low-wavelength part of the spectrum. In particular,filter 620 preferably has a minimum isolation in the drop path for 1528nm to 1536.5 nm of 25 dB and a minimum isolation in the reflection pathfor 1540.5 nm to 1565 nm of 10 dB. Also, the preferred filter has amaximum insertion loss in the reflection path for 1528 to 1536.5 nm of0.7 dB and in the drop path for 1540.5 nm to 1565 nm of 1.5 dB. Otherspecifications for filter 620, of course, will depend on the particularchannel wavelengths and amplifiers chosen for the WDM system 100.

The high band, which is separated from the single wide-band, passes fromfilter 620 to amplifier (AMP) 615. Amplifier 615 is preferably a singleerbium-doped fiber amplifier that is operated in saturation, such thatits output power is substantially independent from its input power. Inthis way, amplifier 615 serves to add a power boost to the channels inthe high band compared with the channels in the low band. Due to thegreater number of channels in the high band compared with the low bandin the preferred embodiment, i.e. twenty-four channels as opposed toeight, the high band channels typically will have had a lower gain whenpassing through the amplifiers for the single wide-band, such asamplifier 610. As a result, amplifier 615 helps to balance the power forthe channels in the high band compared with the low band. Of course, forother arrangements of channels between the high and low bands, amplifier615 may not be required or may alternatively be required on the low bandside of OLA section 130.

With respect to the high band of channels, amplifiers 610 and 615 may beviewed together as a two-stage amplifier with the first stage operatedin a linear mode and the second stage operated in saturation. Relativestabilization of the output power between channels in the low band isgenerally not needed if, as in the presently described embodiment, thenumber of channels in the low band is limited to eight. This may changefor systems having a higher number of channels in the low band. To helpstabilize the output power between channels in the high bands, amplifier610 and 615 are preferably pumped with the same laser diode pump source.In this manner, as described in EP695049, the residual pump power fromamplifier 610 is provided to amplifier 615. Specifically, OLA section130 includes a WDM coupler (not shown) positioned between amplifier 610and filter 620 that extracts 980 nm pump light that remains at theoutput of amplifier 610. This WDM coupler may be, for example, modelnumber SWDMCPR3PS110 supplied by E-TEK DYNAMICS, INC., 1885 Lundy Ave.,San Jose, Calif. (USA). The output from this WDM coupler feeds into asecond WDM coupler (not shown) of the same type and positioned in theoptical path after amplifier 615. The two couplers are joined by anoptical fiber 625 that transmits the residual 980 nm pump signal withrelatively low loss. The second WDM coupler passes the residual 980 nmpump power into amplifier 615 in a counter-propagating direction.

The high band output from booster amplifier 615 then passes throughaffiliated isolators (not shown) and monitoring splitters (not shown)before entering rare-earth doped fiber amplifier 650. For the preferrederbium-doped fiber amplifier, amplifier 650 has a pump wavelength of,for example, 1480 nm from a laser diode source (not shown) having a pumppower in excess of the laser (not shown) that drives amplifiers 610 and615. The 1480 nm wavelength provides good conversion efficiency for highoutput power output compared with other pump wavelengths forerbium-doped fibers. Alternatively, a high power 980 nm pump source or agroup of multiplexed 980 nm pump sources could be used to driveamplifier 650. The amplifier 650 preferably operates in saturation toprovide the power boost to the signals within the high band, and ifdesired, may comprise a multi-stage amplifier.

After passing through amplifier 610 and filter 620, the low band entersequalizing filter 630. As discussed above, the gain characteristic forthe erbium-doped fiber spectral emission range has a peak or hump in thelow band region, but remains fairly flat in the high band region. As aresult, when the low band or the single wide-band, which includes thelow band, is amplified by an erbium-doped fiber amplifier, the channelsin the low band region are amplified unequally. Also, as discussedabove, when equalizing means have been applied to overcome this problemof unequal amplification, the equalizing has been applied across theentire spectrum of channels, resulting in continued gain disparities.However, by splitting the spectrum of channels into a low band and ahigh band, equalization in the reduced operating area of the low bandcan provide proper flattening of the gain characteristic for thechannels of the low band.

FIGS. 7A and 7 B show a graph of the idealized filter shape for the lowband region in equalizing filter 630 and an erbium-doped fiber amplifiergain spectrum with a 980 nm pump, respectively. As shown in FIGS. 7A and7B, the idealized filter shape for equalizing filter 630 and theamplifier gain spectrum are almost exact inverted responses of eachother, and in particular, the peak of the gain characteristic of theamplifier closely corresponds to the bottom of the valley of the filtershape of equalizing filter 630. As a result, the application ofequalizing filter 630 to the low band effectively flattens the gainresponse for the channels in the low band.

In a preferred embodiment, the equalizing filter 630 comprises atwo-port device based on long period chirped Bragg grating technologythat gives selected attenuations at different wavelengths. For instance,equalizing filter 630 for the low band may have an operating wavelengthrange of 1529 nm to 1536 nm, with a wavelength at the bottom of thevalley at between 1530.3 nm and 1530.7 nm. Equalizing filter 630 neednot be used alone and may be combined in cascade with other filters (notshown) to provide an optimal filter shape, and thus, gain equalizationfor the particular amplifiers used in the WDM system 100. Equalizingfilter 630 may be manufactured by one skilled in the art, or may beobtained from numerous suppliers in the field. It is to be understoodthat the particular structure used for the equalizing filter 630 iswithin the realm of the skilled artisan and may include, for instance, aspecialized Bragg grating like a long period grating, an interferentialfilter, or Mach-Zehnder type optical filters, as long as the employedstructure provides the desired filtering response such as that shown inFIG. 7A.

After passing through equalizing filter 630, the low band passes throughanother rare-earth-doped fiber amplifier 640. With the preferrederbium-doped fiber amplifier, amplifier 640 has a pump wavelength of 980nm, provided by a laser diode source (not shown) and coupled via a WDMcoupler (not shown) to the optical path for pumping the amplifier 640 ina counter-propagating direction. Since the channels in the low band passthrough both amplifier 610 and amplifier 640, equalizing filter 630 maycompensate for the gain disparities caused by both amplifiers. Thus, thedecibel drop for equalizing filter 630 should be determined according tothe overall amplification and line power requirements for the low band.The amplifier 640 preferably operates in saturation to provide a powerboost to the signals in the low band, and may comprise a multi-stageamplifier if desired.

FIG. 8A illustrates an experimental result of the output from a WDMsystem with a TPA, four (4) OLAs, a TPA and connecting fibers with six(6) channels in the low band without the use of equalizing filter 630 inOLAs. The graph in FIG. 8A shows how the hump in the low band region ofthe erbium-doped fiber amplifier affects the output power of each of thechannels across that sub-band. The vertical axis of the graph shows arelative monitor output power in dB having 5 dB per division, and thehorizontal axis of the graph shows wavelength in units of nanometers.For example, the channels show a difference in output power of up toabout 15 dB and a signal-to-noise ratio (on a 0.2 nm bandwidth) thatranged from about 22 dB to about 14 dB. FIG. 8B shows the filteringresponse shape of an equalizing filter 630 used for the low band in thesystem test of FIG. 8A. As shown, the equalizing filter 630 used had aminimum passband at around 1530 nm and a maximum at around 1536 nm. FIG.8C depicts the results obtained when combining the equalizing filter 630having the performance of FIG. 8B with the six channels of FIG. 8A. Theuse of the equalizing filter 630 results in a more equal output powerand optical signal to noise ratios for each of the channels within thelow band. Specifically, FIG. 8C shows that the six tested channels had adifference in output power of up to about 8 dB and a signal-to-noiseratio that ranged from about 24 dB to about 20 dB (on a 0.2 nmbandwidth).

FIGS. 8D and 8E illustrate experimental results at the output of a WDMsystem with a TPA, four (4) OLAs, a TPA and connecting fibers with eight(8) channels in the low band without the use of equalizing filter 630 inOLAs. FIG. 8D shows the output response without the use of equalizingfilter 630. The vertical axis of the graph shows a relative monitoroutput power in dB having 5 dB per division, and the horizontal axis ofthe graph shows wavelength in units of nanometers. As shown, the outputpower from amplifier 640 for the test varied from channel-to-channel byup to about 11 dB, while the signal-to-noise ratio (on a 0.2 nmbandwidth) fluctuated from about 13 dB to about 21 dB. In FIG. 8E, theoutput for the eight channels of FIG. 8D are illustrated graphically fora test when the equalizing filter 630 of FIG. 8B was used in the lowband optical path. According to FIG. 8E, with equalizing filter 630, theoutput power of the channels shifted by up to about only 4 dB, while thesignal-to-noise ratio varied from about 18 dB to about 20 dB.

After passing through amplifiers 640 and 650 respectively, the amplifiedlow band and amplified high band are then recombined by filter 660 intothe single wide-band. Like filter 620, filter 660 may also be a low-passthree-port interferential filter. In addition, it is preferred to usefilter 620 as a reflector for the low band and as a transmitter for thehigh band (i.e. high pass filter) and filter 660 in reverse (i.e. lowpass filter) in order to achieve both negligible crosstalk between thebands and optimized output insertion losses for the high band. Like TPAsection 120, OLA section 130 may also include an optical monitor and aservice line insertion and extraction (not shown) through, e.g., a WDM1480/1550 interferential filter (not shown). One or more of theseelements may be included at any of the interconnection points of OLAsection 130.

In addition, OLA section 130 may include several other optical modulesnot shown for optimizing the performance of WDM system 100. Forinstance, the OLA section 130 may include an optical add/drop module(OADM) (not shown) for adding and/or dropping channels from the WDMtransmission path. In a preferred embodiment, an OADM is situatedbetween the output of the second stage amplifier 615 and boosteramplifier 650 for dropping or inserting channels within the highwavelength band. Another OADM may be situated between equalizationfilter 630 and booster amplifier 640 for dropping or inserting channelswithin the low wavelength band. Also, the OLA section 130 may include adispersion compensating module (DCM) (not shown) for compensating forchromatic dispersion that may arise during transmission of the signalsalong the long-distance communication link. The DCM may be incorporatedinto the OADM or at least positioned at the same location as the OADMwithin the high and low band portions of the OLA section 130.

In a preferred arrangement, the OADM includes four-port opticalcirculators, together with gratings and/or interferential filters todirect selected wavelengths. The channels within a particularmultiplexed signal, such as the high band of channels exiting amplifier615, would enter a first port of the optical circulator and rotate tothe next port of the circulator. A series of Bragg gratings havereflection wavelengths corresponding to the channels to be dropped wouldbe coupled to the second port of the circulator. The third and fourthports of the circulator would include additional Bragg gratings tofurther direct the channels to be dropped to the appropriate port.Interferential filters, or the like, may be attached to the outputs ofthe third and fourth circulator ports to further separate and direct theindividual dropped channels. Channels other than those to be droppedwill pass through the Bragg gratings coupled to the second circulatorport and continue in the WDM system 100.

In the OADM described above, the output from the second port of thefirst optical circulator may feed into a first port of a second opticalcirculator. Channels to be added to the WDM system, which shouldcorrespond with the channels that are dropped, may be inserted to thirdor fourth ports of the second optical circulator. When inserted intothese ports, the inserted channels will rotate around the circulator,exit at the first circulator port, be reflected by the Bragg gratingspositioned at that first port, and join the other channels from thefirst circulator port in reentering the first port of the secondcirculator. These combined channels will then rotate around the secondcirculator to the second port and exit for continued transmission alongthe WDM system.

Other arrangements for adding and dropping channels for use with thepresent invention are also acceptable. For instance, an arrangementusing a non-rotating optical device, such as an optical splitter, may beused together with a series of Bragg gratings attached to each of twooutputs from the splitter. The gratings on the first output of thesplitter have reflection bands equal to the channels to be dropped andadded, while the gratings on the second output of the splitter havereflection bands equal to the channels that will not be dropped andadded. A group of interferential filters connected to the output of thegratings on the second splitter output will separate the droppedchannels. A second splitter attached to the output of the gratings onthe first splitter output will insert channels having new information tothe system. Other arrangements for the OADM will be apparent to those ofordinary skill in the art.

The DCM (not shown) may also have several forms. For example, the DCMmay have an optical circulator with a first port connected to receivethe channels in either the high band or the low band. A chirped Bragggrating may be attached to a second port of the circulator. The channelswill exit the second port and be reflected in the chirped Bragg gratingto compensate for chromatic dispersion. The dispersion compensatedsignals will then exit a next port of the circulator for continuedtransmission in the WDM system. Other devices besides the chirped Bragggrating, such as a length of dispersion compensating fiber, may be usedfor compensating the chromatic dispersion. The design and use of theOADM and DCM sections are not restrictive to the present invention andmay employed or omitted in the WDM system 100 depending on overallrequirements for system implementation.

After the OLA section 130, the combined single wide-band passes througha length of long-distance optical transmission fiber. If the length issufficiently long to cause attenuation of the optical signals, i.e. 100kilometers or more, an additional OLA section may be used. In apreferred arrangement, five spans of long-distance transmission fiberare used and separated by four OLA sections.

Following the final span of transmission fiber, RPA section 140 receivesthe single wide-band (SWB) from OLA section 130 and prepares the signalsof the single wide-band for reception and detection at the end of thecommunication link As shown in FIG. 9, RPA section 140 includesamplifiers (AMP) 810, 840, and 850, filter 820, and equalizing filter830, and may further include if needed two router modules 860 and 870.Amplifier 810 comprises a rare-earth-doped fiber amplifier. Thisamplifier 810, which preferably is doped with erbium, amplifies thesingle wide-band with, for example, a 980 nm pump or some otherwavelength to provide a low noise figure for the amplifier, to helpimprove the signal-to-noise ratio. for the channels in the singlewide-band. The single wide-band is in turn separated into the low bandand high band by filter 820. In addition, the low band passes throughequalizing filter 830. As with TPA section 120 and OLA section 130,amplifier 840 amplifies the low band with, for example, a 980 nm pump,and amplifier 850 amplifies the high band with, for example, a 1480 nmpump. Of course, multiplexed 980 nm pump sources or a high power 980 nmpump can be used for driving the high band amplifier as well. Thus,amplifiers 810, 840, and 850, filter 820, and equalizing filter 830perform the same functions as amplifiers 610, 640, and 650, filter 620,and equalizing filter 630, respectively, of OLA section 130 and maycomprise the same or equivalent parts depending on overall systemrequirements.

Other structure may be added to RPA section 140 depending on the channelseparation capability of demultiplexing section 150. If the channelseparation capability of demultiplexing section 150 is for a relativelynarrow channel spacing, e.g. a 100 GHz grid, then the optional channelseparation structure 880 is typically not needed. However, if thechannel separation capability of demultiplexing section 150 is for arelatively wide channel spacing (e.g. 200 GHz grid) while channels inWDM system 100 are densely spaced (e.g. 100 GHz), then RPA section 140could include the optional structure 880 shown in FIG. 9. In particular,RPA section could have channel separation means, such as router modules860 and 870.

Router modules 860 and 870 separate the low band and high band into twosub-bands, each sub-band consisting of half the channels of the band,e.g., with a 200 GHz separation between channels. For example, if thelow band includes eight channels 1-8, each separated by 100 GHz, thenrouter module 860 would split the low band into low sub-band 1 (LSBI)having channels 1, 3, 5, and 7, and low sub-band 2 (LSB 2 ) havingchannels 2, 4, 6, and 8. Although each of the eight wavelengths in thelow band would have a separation of 100 GHz, the router module 860 wouldseparate the odd and even channels so that the channels in each lowsub-band would have double the spacing, i.e. 200 GHz spacing. Routermodule 870 would split the high band into high sub-band 1 (HSB1) andhigh sub-band 2 (HSB2) in similar fashion.

In a preferred arrangement, router modules 860 and 870, in general,include for each band a three-port optical circulator, twelve in-fiberBragg gratings (for 870) in double quantity and four in-fiber Bragggratings (for 860) in double quantity at interleaved wavelengths toimprove isolation, and an optical isolator (all not shown) between thetwin gratings. Each module also requires an optical monitor (not shown)at its output and a fiber grating temperature control (not shown). Inthis configuration, the channels for one of the bands enter a first portof the three-port circulator, rotate within the circulator, and exit ata second port. The second port has a series of Bragg gratings attachedthat have reflection wavelengths corresponding to every other channel inthe band. In this way, every other channel (i.e. every even channel) isreflected, while the remaining channels (i.e. every odd channel) ispassed. The reflected channels re-enter the circulator and then exit ata third port. As a result, the circulator and Bragg gratingconfiguration accomplishes a separation of the channel spacing, and inthis example, takes one input port and creates two output ports withtwice the spacing between channels. Other configurations for the routermodules may be employed, for example, using a WDM coupler that has afirst series of Bragg gratings attached to a first port and a secondseries of gratings attached to a second port. The Bragg gratingsattached to the first port would have reflection wavelengths thatcorrespond to every other channel (i.e. the even channels), while theBragg gratings attached to the second port would have reflectionwavelengths that correspond to the remaining channels (i.e. the oddchannels). This arrangement of gratings will also serve to split thesingle input path into two output paths with twice thechannel-to-channel spacing.

After passing through RPA section 140, the low band and high band ortheir respective sub-bands are received by demultiplexing section 150.As shown in FIGS. 10A and 10B, the structure of demultiplexing section150 depends on the separation capability of its demultiplexers.

FIG. 10A illustrates a preferred embodiment when the WDM system 100 usesa relatively narrow channel separation, e.g. 100 GHz separation. In thissituation, demultiplexing section 150 uses a wavelength demultiplexer(WD) 910 for the low band (LB) and a WD 920 for the high band (HB).Demultiplexing section 150 in FIG. 10A is connected to a plurality ofreceiving units Rx1-Rx32 for receiving each individual channeldemultiplexed by WDs 910 and 920. The individual channels correspond tooutput channels 170, as shown in FIG. 2.

WD 910 in FIG. 10A receives the low band, which includes, for example,eight channels. The low band, with the channels spaced at 100 GHzintervals. as shown in Table 1, is separated into its individualchannels by WD 910, such as a 1×5 type arrayed waveguide grating (AWG)100 GHz demultiplexer. Similarly, WD 920, such as a 1×24 type AWG 100GHz demultiplexer, receives the high band, which includes, for example,twenty-four channels spaced at 100 GHz intervals, and separates the highband into its individual channels. AWG units may be obtained fromvarious suppliers, including Hitachi and PIRI. Output channels 170 arecomposed of the individual channels produced by WD 910 and 920. Eachchannel of output channels 170 is received by a respective receivingunit. Receiving units Rx1-Rx32 represent any kind of port, connection,or processing means that is coupled to receive a signal from aparticular channel.

FIG. 10B illustrates a configuration for a WDM system 100 that has analignment of channels such as 100 GHz spacing. This arrangement may beused in conjunction with router modules 860 and 870 that separate thelow band and high band into two sub-bands each, such that each sub-bandincludes half the channels of the corresponding band with a 200 GHzseparation between channels. In particular, demultiplexing section 150in FIG. 10B includes four WDs 930, 940, 950, and 960, although dependingon economic and commercial factors, demultiplexing section 150 couldinclude one for each sub-band. Again, the wavelength demultiplexerspreferably comprise arrayed waveguide grating devices, but alternatestructures for achieving the same or similar wavelength separation arecontemplated. For instance, one may use interferential filters,Fabry-Perot filters, or in-fiber Bragg gratings in a conventional mannerto demultiplex the channels within the low band (LB), the high band(HB), the low sub-bands (LSB), and the high sub-bands (HSB). Like inFIG. 10A, demultiplexing section 150 also includes receiving unitsRx1-Rx 32 for receiving output channels 170.

In a preferred configuration, demultiplexer section 150 in FIG. 10Bcombines both interferential filter and AWG filter technology. In thismanner, WDs 930 and 940, which-are preferably four channeldemultiplexers with interferential filters, receive and demultiplex lowsub-band 1 and low sub-band 2, respectively. Specifically, WD 930produces channels 1, 3, 5, and 7, and WD 940 produces channels 2, 4, 6,and 8. Similarly, WDs 950 and 960 receive and demultiplex high sub-band1 and high sub-band 2, respectively, to produce channels 9-32. Both WD950 and WD 960, however, may be 1×16 type AWG 200 GHz demultiplexersthat are underequipped to use only twelve of the available sixteendemultiplexer ports. Output channels 170 are composed of the individualchannels produced by WDs 930, 940, 950, and 960, and each channel ofoutput channels 170 is received by one of receiving units Rx1-Rx32.

FIG. 11A illustrates an experimental result from a portion of the WDMsystem 100 using the multi-band amplification scheme of the presentinvention. In the setup for FIG. 11A, a WDM system was arranged usingTPA 120 and RPA 140, together with four OLA 130 sections positionedbetween five transmission fiber spans. As shown in FIG. 11A, only thehigh band (HB) was employed, i.e. amplifier 650 in the OLA 130 sectionsand amplifier 850 in RPA 140. The graph depicts the results oftwenty-one (21) channels across the high band of a total thirty-two (32)channel system, although the total power was equivalent to twenty-four(24) channels across the high band. As can be seen from FIG. 11A, thehigh band provides a relatively constant signal level and opticalsignal-to-noise ratio from channel to channel. For instance, thesignal-to-noise ratio varied between about 17 dB and about 21 dB. Thelarge hump evident in the area of the low band is amplified spontaneousemission generated by the amplifiers for the channels that were notapplied during the test of the system.

FIG. 11B illustrates a similar test result to FIG. 11A with the sametest setup, except where only the low band channels were used. In otherwords, equalizing filter 630 and amplifier 640 in the OLA 130 sectionsand equalizing filter 830 and amplifier 840 in RPA 140 were used but thetransmitters for the high band channels were not turned on. The graphdepicts the results of six (6) channels across the low band of a totalthirty-two (32) channel system, although the total power was equivalentto eight (8) channels across the low band. As can be seen from FIG. 11B,the low band provides a relatively constant level and optical signal tonoise ratio from channel to channel. For instance, the signal-to-noiseratio varied between about 24 dB and about 20 dB. The large hump evidentin the area of the high band is amplified spontaneous emission generatedby the amplifiers for the channels that were not applied to the systemduring the test.

Finally, FIG. 11C shows the results of the same test setup of a fivespan system where both the low band and the high band were operating.FIG. 11C depicts the separation between the high band and the low band,with an unused section between about 1536 nm and 1541 nm that has aspectral emission trough. As can be seen from FIG. 11C, the individualchannels in both the low band and the high band with the presentinvention provide a relatively constant output level, which leads toimproved detection by the receivers and more reliable transmission fordense wavelength spacing. Moreover, the results in FIG. 11C, whencompared with those of FIG. 11A, show that the presence of the low bandtogether with the high band does not affect the output of the high bandchannels compared to when the high band channels are used alone in thesystem. Similarly, a comparison of FIG. 11C with FIG. 11B reveals thatthe presence or absence of the high band channels does not affect theoutput power of the low band channels, Thus, the system of the presentinvention using multi-band amplification provides both gain equalizationfor dense wavelength spacing and relative subband channel independenceand robustness. Further, gain tilts in the two bands are independentfrom each other, and generally smaller than achieved before, thusensuring a higher span loss dynamic range or, in other words, broadeningthe range of span loss values that is acceptable for the system.

FIG. 12 is a chart of the preferred maximum span attenuation in dB forvarious system configurations of WDM system 100. This graph includes theattenuation values applicable to the tests of FIGS. 11A, 11B, and 11Cthat are reported above for a thirty-two channel system having fivetransmission spans.

The optical transmission system consistent with the present inventiontherefore optimizes the use of the entire erbium-doped fiber spectralemission range by separating the range into a low band corresponding tothe low end of the range and a high band corresponding to the high endof the range. Since the low band suffers from unequal amplification, theuse of an equalizing filter in the low band region flattens the gain forthe channels in the low band without resulting in residual gainundulations caused by the application of equalization to the entirerange of channels. Also, the use of a first unsaturated amplifierfollowed by the separation of the wide band into subbands provides arelative independence between the amplification and power of thechannels in the sub-bands.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to disclosed embodiments of thepresent invention without departing from the scope of the invention. Forexample, while described in terms of the wavelength band forerbium-doped fiber amplifiers, the present invention applies equally tothe wavelength band for other rareearth-doped fiber amplifiers anddoping compositions as well and also to other pumping schemes andpumping conditions. Moreover, the system consistent with the presentinvention may include the separation of the single wide band intomultiple sub-bands numbering greater than two. Other embodiments of theinvention will be apparent to those skilled in the art fromconsideration of the specification and practice of the embodiments ofthe invention disclosed herein. The specification and examples should beconsidered exemplary, with the true scope of the invention beingindicated by the following claims and their full range of equivalents.

What is claimed is:
 1. An optical communication system for transmittingoptical channels between a transmitter and a receiver using wavelengthdivision multiplexing, comprising: a wavelength multiplexer opticallycoupled to the transmitter for multiplexing individual optical channels;a transmitter power amplifier optically coupled to the wavelengthmultiplexer for amplifying the multiplexed optical channels; at leastone optical line amplifier being optically coupled to the transmitterpower amplifier via an optical transmission fiber; a receiverpre-amplifier optically coupled to the at least one line amplifier viaanother optical transmission fiber; and a wavelength demultiplexeroptically coupled to the receiver pre-amplifier for separating themultiplexed optical channels into the individual optical channels forpassage to the receiver; said optical communication system characterizedin that the at least one optical line amplifier includes: a first stageof a first fiber amplifier for amplifying the multiplexed opticalchannels; a first band separation filter optically coupled to an outputof the first stage for splitting the multiplexed optical channels into afirst band of wavelengths and a second band of wavelengths, each of saidbands covering a range of at least 6 nm; a second stage of the firstfiber amplifier optically coupled to the band separation filter; asecond fiber amplifier optically coupled to the band separation filterand having a first wavelength response characteristic for amplifying thefirst band; an equalizing filter, positioned between the band separationfilter and the second amplifier, for equalizing the amplification ofsignals in the first band; a third fiber amplifier optically coupled tothe second stage and having a second wavelength response characteristic,different from the first wavelength response characteristic, foramplifying the second band; and a combiner for multiplexing the firstamplified band and the second amplified band back into the multiplexedoptical channels.
 2. An optical line amplifier for amplifying aplurality of multiplexed channels traveling in a wavelength divisionmultiplexing system, comprising: a first optical amplifier, opticallycoupled to receive the multiplexed channels, having a first stageoperating in a linear mode and a second stage operating in a saturationmode; a band separation filter positioned between the first stage andthe second stage for passing a first group of the multiplexed channelsinto the second stage and separating a second group of the multiplexedchannels from entering the second stage; a second optical amplifier,optically coupled to an output of the second stage, having a firstwavelength response characteristic for amplifying the first group of themultiplexed channels; a third optical amplifier, optically coupled tothe band separation filter, having a second wavelength responsecharacteristic different from the first wavelength responsecharacteristic for amplifying the second group of the multiplexedchannels; and an equalization filter positioned between the bandseparation filter and the third optical amplifier for flattening thegain response of the third optical amplifier for the second group of themultiplexed channels.
 3. A method for transmitting optical signals,comprising the steps of: amplifying a multiplexed signal having aplurality of optical channels with a first stage of a first amplifieroperating in a linear condition; splitting the multiplexed signal into afirst wavelength band and a second wavelength band, each of said bandscovering a range of at least 6 nm; amplifying the first wavelength bandwith a second stage of the first amplifier operating in a saturationcondition; amplifying the first wavelength band after the second stagewith a second amplifier having a first wavelength responsecharacteristic; filtering the second wavelength band to flatten a gainresponse; and amplifying the second wavelength band with a thirdamplifier having a second wavelength response characteristic differentfrom the first wavelength response characteristic.
 4. The opticalcommunication system according to claim 1, wherein the individualoptical channels includes thirty-two signals, wherein the first band ofwavelengths includes eight of the thirty-two signals and the second bandof wavelengths includes twenty-four of the thirty-two signals.
 5. Theoptical communication system according to claim 1, wherein the firstband of wavelengths includes 1529 nm to 1535 nm and the second band ofwavelengths includes 1541 nm to 1561 nm.
 6. The optical communicationsystem according to claim 1, wherein the transmitter power amplifiercomprises a fourth fiber amplifier for amplifying the first band ofwavelengths, a fifth fiber amplifier for amplifying the second band ofwavelengths, and a second combiner having a first input coupled to anoutput of the fourth amplifier and a second input coupled to an outputof the fifth amplifier, for multiplexing the first amplified band andthe second amplified band into the multiplexed optical channels.
 7. Theoptical communication system according to claim 6, further comprising ade-emphasis filter positioned between the wavelength multiplexer and thesecond input of said second combiner for equalizing the amplification ofsignals in the second band.
 8. The optical communication systemaccording to claim 7, wherein the fourth amplifier is pumped with lightat 980 nm, and wherein the fifth amplifier is pumped with light at oneof 980 nm and 1480 nm.
 9. The optical communication system according toclaim 1, wherein the first stage of the first fiber amplifier operatesin a linear mode.
 10. The optical communication system according toclaim 9, wherein the second stage of the first fiber amplifier operatesin a saturation mode.
 11. The optical communication system according toclaim 1, wherein residual pump light from the first stage of the firstfiber amplifier is used to pump the second stage of the first fiberamplifier.
 12. The optical communication system according to claim 1,wherein the wavelength multiplexer includes a plurality of wavelengthconversion modules each including a photodiode for converting a receivedsignal to an electrical signal, an optical source for originating anoptical carrier signal, and an electro-optic modulator for modulatingthe optical carrier signal with the received signal to produce one ofthe individual optical channels.
 13. The method according to claim 3,further comprising the step of: combining the first wavelength band fromthe second amplifier and the second wavelength band from the thirdamplifier back into the multiplexed signal.
 14. The method according toclaim 3, wherein the steps of amplifying include the substeps of:providing pump energy at a pump wavelength to the first stage; androuting to the second stage residual pump energy not used by the firststage.